Although there are various methods for separating specific component gas (product gas) such as nitrogen and oxygen from a mixed gas such as air, a method of separation using an adsorbent has been recently employed widely for its ease of designing an apparatus and low cost of equipment. Such a method of separation using an adsorbent is generally called the PSA method, wherein plural adsorption towers are filled with adsorbent, and operations such as supply of a mixed gas to these adsorption towers, adsorption of specific component gas, desorption of specific component gas, and reactivation of the absorbent are conducted by alternately switching of valves. The supply of material gas is conducted in pressurized condition and desorption of the specific component gas is conducted in normal pressure or by vacuum suction. In an apparatus for the PSA method, the number of times in which the opening and closing valves are required is extremely high, since the above-mentioned operations are conducted one after another, which shortens life duration of the valves, and, at the same time, the intensively frequent switching of the valves increases fluctuations of pressure in the adsorption towers. As a result, a difficult point arises, that is, an increment of fluctuations of the purity of the specific component gas. Also, when the pressure is equalized and the like, the occurrence of noise by the instantaneous inflow of large quantity of gas is unavoidable. Moreover, each adsorption tower is required to be arranged on a same plane in the apparatus, so that the apparatus requires a very large space.